Nutritional status has a complex relationship to health and functional status in the elderly. Older persons are at high risk for morbidity from malnutrition due to a combination of age-related physiological changes as well as the development of acute and chronic illnesses. Marginal protein nutritional status, in particular, is of concern in the elderly, as it may contribute to detrimental changes in body composition and organ function with advancing age, as well as increasing susceptibility to acute illness. Plans are to study the nutritional status of elderly patients with chronic renal failure who are newly prescribed a low protein diet as a means to slow progression of their renal disease, as an ideal model of chronic marginal protein nutrition. In addition, plans are to investigate the efficacy and mechanism of resistance training as an anabolic stimulus which may counteract the catabolic effects of protein undernutrition. The hypotheses which will be tested are: (1) Free living elderly individuals with early stage chronic renal failure following a low protein diet of about 0.6 g per kg body weight per day will demonstrate significant losses in body cell mass and muscle cross-sectional area, which are associated with decreased muscle function, insulin-like growth factor (IGF-1), and whole -body protein turnover as assessed by leucine kinetics. (2) A progressive resistance exercise training program will attenuate these declines in body cell mass, muscle cross-sectional area and function seen in diet-only controls. (3) The mechanism by which progressive resistance exercise training will improve nitrogen economy will be via an increase in whole-body protein turnover as assessed by rates of leucine flux and leucine oxidation, in association with elevation of plasma levels of IGF-1, and the expression of IGF-1 in skeletal muscle. The design of this study is a randomized controlled trial of 30 subjects over the age of 50 with early to moderate stage chronic renal failure. Subjects will be randomized to receive a low protein diet of 0.6 g/kg body weight/day plus stretching exercises or the same diet plus whole-body resistance training 3 days/ week for 12 weeks. The effects of the diet and exercise interventions on lean tissue mass will be measured by whole- body potassium, computerized tomography scans of the thigh muscles, and skeletal muscle fiber cross-sectional area. Whole body protein turnover will be assessed by leucine kinetics. The functional outcomes of changes in nitrogen economy will be assessed via dynamic muscle strength, leg power output, and the contraction-relaxation characteristics of the adductor pollicis muscle.